1. Field of the Invention
The present invention relates to an apparatus for demodulating a Sub-Nyquist sampled video signal and a demodulating method therefor, and more particularly, it relates to an apparatus for demodulating a Sub-Nyquist sampled video signal to include no aliasing noise component caused by Sub-Nyquist sampling in its low-frequency range and a demodulating method therefor. More specifically, the present invention relates to circuit structure and a method for compensating dropout in a received Sub-Nyquist sampled video signal.
2. Description of the Background Art
A band compressing technique called MUSE (Multiple Sub-Nyquist Sampling Encoding) is a system of transmitting a television signal of high picture quality. For example, NHK Research Laboratory Monthly Review, July 1984, pp. 275 -285 discloses a method of encoding and decoding a signal in such a MUSE system. The signal compressing technique in this MUSE system is now briefly described. In the MUSE system, an HDTV (high definition television) signal having 1215 scanning lines is transmitted through a single channel of 27 MHz in bandwidth. The baseband width is compressed to about 8 MHz in order to transmit an FM-modulated high definition signal in the single channel of 27 MHz in bandwidth. FIG. 1 shows the sampling pattern in such a MUSE system. As understood from FIG. 1, this sampling is multiple interlace type Sub-Nyquist sampling with a cycle of four fields. As shown in FIG. 1, signals are culled out to be sampled according to this sampling technique, with 180.degree. sampling phase difference between lines and between fields. Thus, required bandwidth for transmission converted in spatial frequency is 1/(4d) (see FIG. 1). In the MUSE system, the required bandwidth for transmission is set at 8.1 MHz. That is, the transmission sampling rate is 16.2 MHz. According to this sampling pattern, successively transmitted signals are so sequentially stored in a receiving (reproducing) side that pictures can be reproduced by using all of the sampling points shown in FIG. 1 in the case of a still picture area. FIG. 2 shows a transmissible region of the spatial frequency domain for a still picture area (portion of the field where the picture is still). Referring to FIG. 2, the horizontal axis represents horizontal spatial frequency and the vertical axis represents vertical spatial frequency. As shown in FIG. 2, values of vertical and horizontal spatial frequencies are 1/(2h)=1125 TV lines and 1/(2d)=32.4 MHz in the case of the still picture area.
In the case of a motion picture area (portion of the field where the picture moves), multi-line blur is caused or the sampling pattern appears on the scene in the form of a net if past sampling points are used. Hence, the picture must be reproduced by using only the sampling points of the current field. As shown in FIG. 3, a transmissible region in this case is narrower than that in the still picture area, and hence blurring of the picture is noticeable in the case of motion caused by panning or tilting. To avoid such influence, spatial interpolation and motion compensation are performed. An encoder calculates a vector (motion vector) representing the motion of a scene for each field. A motion vector signal is multiplexed in a vertical blanking period of a video signal and transmitted to a receiver as a control signal. In response to the motion vector, a decoder provided in the receiver shifts and overlaps positions of past picture data (those of a preceding field, for example) to process the same as those of a still picture area. Since the picture data, which is processed as that of a motion picture area when no motion correction is performed, is processed as that of a still picture area, resolution of the picture can be improved. In the MUSE system, motion correction is performed only when the entire scene uniformly moves in the case of panning, for example. With such motion correction, temporal interpolation is applied to a panned or tilted scene. As shown in FIG. 3, the maximum vertical transmissible frequency for a motion picture area is half that for a still picture area. This is because the original high definition television (HDTV) signal is subjected to 2:1 interlace scanning. If spatial interpolation is applied to a still picture area of a scene, the maximum transmissible vertical spatial frequency is doubled to 1/(2h), where h represents an interval between adjacent horizontal scanning lines.
In the aforementioned sampling system, aliasing noise is caused around a sampling frequency through Sub-Nyquist sampling (hereinafter simply referred to as subsampling). When such aliasing noise is caused, noise conspicuously appears on the scene. Thus, it is desirable to remove such aliasing noise, particularly that extending toward a low frequency range.
In the aforementioned multiple subsampling transmission system, a subsampling cycle consists of two frames (four fields). Hence, one-interframe difference signal cannot be used for motion detection on a receiver side (decoder side) since there is no object to be detected but two interframe difference signal must be employed. Thus, motion detection is imperfect.
The term "interframe difference" indicates signal level difference between first and second frames in FIG. 1, for example, and the term "difference between next adjacent frames" indicates signal level difference between the first and third frames in FIG. 1, for example. The reason why this motion detection is imperfect is now described in more concrete terms. As to a still picture area, interpolation is performed by using a signal in a preceding frame. However, such interpolation cannot be applied to a motion picture area.
Thus, there has been proposed a multiple subsampling transmission system which performs motion detection by using a complete interframe difference signal for simplifying the structure of a receiver (decoder) and improving the picture quality. Such improving system is disclosed in U.S. Pat. No. 4,692,801 and in Nikkei Electronics, November 2, No. 433, pp. 189 to 212. The methods disclosed in these references are both a technique for generating no aliasing noise or reducing it basically. The method of this improved MUSE system is now briefly described, with reference to U.S. Pat. No. 4,692,801. According to this method, sampling is so performed as to cause no aliasing noise in a low-frequency component, and the low-frequency component having no aliasing noise is employed as a signal for detecting one-interframe difference signal.
This system is now briefly described with reference to the drawings. In the following description, the term "interframe/interline offset subsampling" indicates subsampling carried out by using clocks which are inversed every frame/line, in correspondence to sampling points in the 4n-th field and the (4n+2)-th field shown in FIG. 1, for example.
The term "interfield offset sampling" indicates subsampling carried out by using clocks which are inversed in phase every field, in correspondence to sampling points in the 4n-th field and the (4n+2)-th field, and those in the fourth field, the (4n+1)-th field and the (4n+3)-th field in FIG. 1, for example.
Further, the term "interframe in-phase" indicates the fact that, when a high-frequency component is aliased, the amplitude of a corresponding signal (8-12 MHz, for example) is in phase in succeeding frame. This definition also applies to "interfield in-phase".
The term "temporal interpolation" indicates interpolation processing performed through sampling values between picture signals having time difference such as interfield difference (time difference: 1/60 sec.) or interframe difference (time difference: 1/30 sec.). With reference to FIGS. 4A to 4F, brief description is made on encoding and decoding in the MUSE system including no aliasing noise in the low-frequency range.
(1) An input signal is sampled at a sampling frequency of 48.6 MHz. Thus, an output having bandwidth as shown in FIG. 4A is obtained Referring to FIG. 4A, the horizontal axis represents horizontal frequency component and the vertical axis represents signal level.
(2) Filtering processing is performed by an interfield prefilter (not shown) in order to process a still picture portion, whereby high frequency components in oblique directions are removed as shown in FIG. 4B.
(3) Interfield offset sampling is carried out at a sampling frequency of 24.3 MHz. Consequently, signals in a frequency range exceeding 12.15 MHz are aliased about the frequency of 12.15 MHz, as shown in FIG. 4C.
(4) The sampling frequency of 24.3 MHz shown in FIG. 4C is converted to that of 32.4 MHz by sampling frequency conversion processing. In this case, the signal bandwidth remains in that shown in FIG. 4C. In other words, only the sampling frequency is converted while the frequency bandwidth remains unchanged.
(5) Filtering processing is performed by a filter having a characteristic shown in FIG. 4D for processing a motion picture portion, thereby to limit the frequency bandwidth of the horizontal component to 12 MHz.
(6) The bandwidth-limited signal is further sampled at the sampling frequency of 24.3 MHz. In this case, no aliasing is caused since the sampling frequency is 24.3 MHz and the frequency bandwidth of the bandwidth-limited signal is limited to 12 MHz, and hence the signal bandwidth shown in FIG. 4D is maintained.
(7) The signal sampled at the sampling frequency of 24.3 MHz as shown in FIG. 4D is subjected to sampling frequency conversion, so that its sampling frequency is converted to 32.4 MHz. In this case, only the sampling frequency is converted and the signal bandwidth remains in that shown in FIG. 4D. Since no change is caused in the signal bandwidth at the steps (6) and (7), the sampling frequency is directly converted from the original frequency of 48 MHz to 32.4 MHz.
(8) A difference signal between adjacent frames is detected and the absolute value of the frame difference signal is obtained. This absolute value is converted to a nonlinear motion detection signal through a ROM, for example, and outputted as a signal indicating the amount of motion.
(9) In accordance with the amount of motion thus obtained, the still picture obtained at the step (4) and the motion picture obtained at the step (7) are mixed with each other at a ratio responsive to the amount of motion.
(10) The composite signal is subjected to interframe offset sampling at a sampling frequency of 16.2 MHz. Consequently, the still picture (FIG. 4C) and the motion picture (FIG. 4D) are aliased in a 8.1 MHz region, to have bandwidth values shown in FIGS. 4E and 4F respectively. As shown in FIG. 4D, the bandwidth of the motion picture area is limited to 12.15 MHz, and hence no aliased portion is present in a frequency range lower than 4 MHz
(11) Thereafter a digital-to-analog converted composite signal is transmitted.
On the decoder side, procedure reverse to the above is performed.
Demodulating operation on the decoder side is now described.
(1) First, an analog-to-digital converter performs re-sampling. Thus, the signal bandwidth of the still picture is brought into that shown in FIG. 4E, and the signal bandwidth of the motion picture area is brought into that shown in FIG. 4F.
(2) The still picture is subjected to interframe pixel insertion. That is, non-sampled pixels are replaced by those in a preceding frame. Consequently, the signal spectrum shown in FIG. 4C is reproduced from an aliased portion shown in FIG. 4E. The term "interframe interpolation" indicates procedure for obtaining an interpolation signal by using sampled values obtained from succeeding frames.
(3) The signal shown in FIG. 4C is subjected to sampling frequency conversion. Thus, the sampling frequency is converted from 32.4 MHz to 24.3 MHz while the signal bandwidth remains unchanged.
(4) Interfield interpolation processing is performed on the signal subjected to sampling frequency conversion, whereby a signal having the signal bandwidth shown in FIG. 4B is reproduced from the aliased portion shown in FIG. 4C.
(5) The motion picture area is subjected to interfield interpolation processing. Thus, the aliased portion shown in FIG. 4D is reproduced from a frequency spectrum shown in FIG. 4F.
The term "interfield interpolation" indicates procedure for obtaining an interpolation signal by using sampled values in the same field.
(6) Sampling frequency conversion is performed on the motion picture area, whereby the sampling frequency is converted from 32.4 MHz to 48.6 MHz. In this case, the signal bandwidth remains in that shown in FIG. 4D.
(7) Bandwidth limitation is performed on the motion picture area, so that its signal bandwidth is limited to 4 MHz. An interframe difference signal is obtained by the bandwidth-limited signal. This interframe difference signal is then subjected to nonlinear processing, to provide an amount of motion.
(8) In accordance with the detected amount of motion, the still picture area and the motion picture area are linearly mixed with each other.
(9) Digital-to-analog conversion processing is performed. In this case, the still picture has the signal bandwidth shown in FIG. 4B while the motion picture area has the signal bandwidth shown in FIG. 4D.
As hereinabove described, a difference signal between adjacent frames (one interframe difference signal) is derived by using a signal including no aliased signal component in its low-frequency range, and this difference signal is obtained as a signal for detecting motion (step 7). Correct motion detection is effected by such one interframe difference signal including no aliasing noise.
In the aforementioned structure, a frame memory is required for the decoder, in order to derive the difference signal between adjacent frames. Thus, it is possible to perform noise reduction through the frame memory, to reduce required C/N (carrier/noise) in a transmission path. However, the quality of a still picture is generally degraded upon noise reduction, and hence such noise reduction is avoided with respect to a motion picture area. In such structure, however, a motion picture may be judged as a still picture particularly in a flat portion of the picture due to incomplete judgement of motion/stillness, to cause a delay in the motion of this portion. Thus, motion of the entire scene may be ununiformalized upon movement of a camera, for example.
A similar phenomenon may be caused in signal decoding by the multiple subsampling system itself, which includes no interframe aliased component in the low frequency range. This is because, in such signal decoding, temporal interpolation is employed for a still picture area and spatial (intrafield) interpolation is employed for a motion picture area, and hence a delay is caused in motion of a flat portion due to an error in judgement of motion/stillness.
In order to solve the aforementioned problem, a low-frequency range (lower than 4 MHz) including no interframe aliased component is utilized to remove incompleteness in motion correction in the aforementioned detection of the motion picture area and to improve vertical resolution. That is, a low-frequency component of composite data is replaced by that of a given MUSE signal in signal demodulation. In this case, the low-frequency component is not subjected to interframe interpolation, interfield interpolation and intrafield interpolation operation, and hence a signal subjected to no filtering processing in the vertical direction is used as a transmission component, whereby it can be expected to obtain superior quality with respect to vertical resolution.
In a decoder of the aforementioned method of replacing the low-frequency range, however, the following problem is caused: In the case of reproducing a video signal from a recording medium for a high definition video signal such as a video disk, dropout may be caused in a reproduced MUSE signal by a flaw in the disk or the like. In this case, MUSE signal input is blocked upon generation of dropout detection output, and the dropout portion is replaced by a reproduced MUSE signal preceding by two frames, thereby to compensate the dropout portion in the conventional system, as described in Japanese Patent Laying-Open Gazette No. 56584/1986, for example.
If the conventional structure is applied to the high definition video signal decoder of the low-frequency replacement system to perform the aforementioned replacement of the low-frequency components when dropout is caused in a reproduced MUSE signal, a low-frequency component of high definition video data, which must be dropout-compensated by pixel data preceding by four fields and derived, is replaced by reproduced data including the dropout. Consequently, no dropout compensation is performed and a noise component with the dropout appears on the monitor screen.